Polyacrylate functional materials are widely used in coating compositions curable with mechanisms initiated by free radicals (UV/EB, peroxide, etc) or other reactive species, but there are only a limited number of structural variations available as articles of commerce. These materials are currently manufactured primarily by reaction between acrylic acid and polyol or polyepoxy compounds, forming polyacrylic acid esters. While polyol and polyepoxy materials with broad structural variety are generally available articles of commerce, acrylic acid is a corrosive and hazardous material, requiring special handling procedures, rendering this process a specialized conversion. Furthermore, because of the volatility of acrylic acid and its tendency to thermal polymerization, the esterification reaction must be conducted at temperatures below about 125° C., limiting the reaction rate. In some cases, these difficulties may be avoided by transesterification of polyols by acrylic acid esters of the lower alcohols with removal of volatile reaction products to drive the reaction. Although this circumvents the problems associated with handling acrylic acid, it introduces some new ones, which may be more difficult or expensive to overcome. These new problems include handling volatile acrylic monomers, which are both toxic and skin sensitizers, and installation of efficient fractionation equipment to separate the reaction products to be removed, generated during transesterification, from the volatile starting monomers. Also known are methods to make polyacrylate functional compounds from reactions between polyisocyanate containing compounds and hydroxyl containing acrylate esters to make polyurethane polyacrylate materials. However, commodity polyisocyanates are toxic, water sensitive and relatively expensive starting materials with limited structural availability, reducing the appeal of this approach as well.
The Michael addition reaction between low molecular weight C—H acidic donor compounds and electron deficient C═C acceptor compounds is a well-known reaction in the literature of organic chemistry (for example, Choudary, et al., Green Chemistry 3, 257, 2001). A number of catalysts are similarly known to be effective for this reaction (for example, Kotsuki, et al., J. Org. Chem., 64, 3770, 1999 and Bartoli, et al., Eur. J. Org. Chem., 617, 1999, as well as many others), and many have been described (U.S. Pat. No. 5,539,017 and U.S. Pat. No. 5,496,896) as useful in thermal crosslinking of coatings consisting of polyacetoacetate and polyacrylate containing compounds. Ashland, (WO 01/00684 (2001), U.S. Pat. No. 6,025,410 (2000), U.S. Pat. No. 5,945,489 and USP application 20040072979) has recently disclosed using strongly basic soluble catalysts at moderate temperatures to react C—H acidic Michael donor compounds, including beta-dicarbonyl containing resins and nitroaliphatic compounds, with excess low molecular weight polyacrylates to form polyacrylate containing polymers dissolved in polyacrylate diluents in situ for use as ultraviolet light and peroxide curable compositions.
There are, however, drawbacks to these Ashland approaches, in that the materials they describe have only limited storage stability. The unsatisfactory storage stability of the Ashland materials can be attributed to both incomplete reaction between the polyacetoacetates and polyacrylates and to the presence of active catalyst in the final product mixtures.
Furthermore, in order to make liquid compositions suitable for coatings applications in the Ashland patents, a large molar excess of the acrylate C═C groups is generally required, and as the number of acetoacetate or other donor groups on the resin increases, the excess of acrylate C═C acceptor groups must be increased substantially. Additionally, this excess must be significantly larger if the polyacrylate contains more than two C═C groups. Since such large molar acrylate C═C group excesses are required to produce low viscosity liquid compositions, only a minor amount of the acrylated resins formed in situ will be incorporated in the coatings compositions, limiting their contribution to the cured properties obtained. Therefore, most of the properties attained by the final cured coating will be attributed to the cured properties of the polyacrylate acceptors. Because the polyacrylate acceptors contain 2 or more C═C groups per molecule and generally have low equivalent weights per C═C, they tend to give brittle coatings with high shrinkage and poor adhesion when fully cured.
A more recent prior art (European Patent Application 1431320) discloses an improvement over the Ashland art wherein the excess of polyacrylate required to produce a liquid composition is reduced by first reacting the polyacetoacetate resin with a monofunctional Michael acceptor and then reacting the residual acidic C—H sites in the polyacetoacetate with a polyacrylate in the presence of a basic catalyst. As in the Ashland prior art approach, a disadvantage of this approach is the difficulty in controlling the molecular weight of the polyacrylate product resin, producing high viscosity products that still require substantial quantities of diluents to make useful coating compositions.
Also, the Michael reaction between polyacetoacetates and polyacrylates is generally faster than the standard esterification of polyols by acrylic acid, and is very exothermic. As a result, once it has begun, it can be difficult to control the reaction temperature at a commercially useful scale (and in the absence of solvent), potentially leading to the dangerous condition of a runaway reaction.
U.S. Pat. No. 6,657,036 discloses the use of heterogeneous catalysts like ion exchange resins in the preparation of polycondensation resins and polyaddition resins such as polyamides, polyesters, polycarbonates or urea resins. This process describes the polycondensation and polyaddition of low molecular mass compounds permitting the reaction rate to increase without problems using catalysts without thermal damage to the polyaddition resins or polycondensation resins formed. The catalysts employed for this process no longer induce unwanted secondary reactions and are easy to remove after the final step of the reactions. However, this reference does not teach the usefulness of using these catalysts in Michael reactions, nor solely the more than one functional group per molecule of reactants, the absence of monofunctional blocking groups as well as the molar ratios of said reactants in preparing a final product in a liquid medium and with improved properties. Most importantly, this reference does not recognize the unique product distribution obtainable by the practice of the present invention, in particular for molecules with highly branched structures.
Accordingly, there is a need for a class of catalysts for the Michael reaction that do not require either a large excess of polyfunctional Michael acceptors or monofunctional blocking groups to provide useful liquid compositions. It is also important to have a means of controlling product molecular weight separate and distinct from simple statistics through use of a large excess of one of the reactants. In addition, it is desirable not to have active catalyst residues present in the final product which can affect its long term storage stability.